At the beginning of your upkeep, you may sacrifice a Forest. If you sacrifice a snow Forest this way, Gargantuan Gorilla gains trample until end of turn. If you don't sacrifice a Forest, sacrifice Gargantuan Gorilla and it deals 7 damage to you.

: Gargantuan Gorilla deals damage equal to its power to another target creature. That creature deals damage equal to its power to Gargantuan Gorilla.

If this leaves the battlefield before its activated ability resolves, it will still deal damage to the targeted creature. On the other hand, if the targeted creature leaves the battlefield before the ability resolves, the ability will be countered and no damage will be dealt.